Rubber compositions for tire treads, tire sidewalls or the like contain wax or other additives for preventing degradation by ozone or by oxidation. The added wax migrates to the rubber surface to form a film on the rubber surface, which physically protects rubber from physical stimuli of ozone, oxygen, and other harmful gases.
In cold to temperate regions, wax blooming is less likely to occur, which makes it difficult to ensure ozone resistance. Thus, the use of conventional wax in an amount that allows prevention of discoloration may result in insufficient ozone resistance in cold environments (for example, in winter in the temperate regions). In hot regions, on the other hand, wax blooming is likely to occur because rubber molecules are in active motion. This facilitates ensuring ozone resistance but easily causes the surface of tires to turn white. Therefore, it is difficult to provide excellent ozone resistance over a wide temperature range from intense-cold to tropical zones while preventing white discoloration.
In particular, silica-containing tread rubbers are likely to be attacked by ozone because the rubbers are likely to shrink by vulcanization so that tension is applied to the bottom of the tread grooves. Accordingly, tread groove cracking (TGC) is likely to occur. Thus, it is important to ensure both ozone resistance and discoloration resistance.
Another known approach for improving ozone resistance is to use the antioxidant 3PPD, which tends to migrate quickly to the surface. This agent is capable of improving ozone resistance in cold environments because it easily blooms as compared to 6PPD or 6QDI. However, it is difficult for this agent to ensure ozone resistance for a long period of time because it is highly volatile and will disappear in a short time.
Patent Literature 1 proposes the use of a certain wax. This method, however, has much room for improvement in terms of ozone resistance in cold regions or in winter in the temperate regions. Therefore, there remains a need for techniques that can provide excellent ozone resistance over a wide temperature range while preventing discoloration.
Meanwhile, in the case of inner layer components of tires, for example, a technique for a base tread is known which uses wax in an amount equal to that used for a cap tread, and an antioxidant in an amount equal to or more than that used for the cap tread. They are used to prevent tread groove cracking (TGC) as TGC is likely to occur in markets when the minimum rubber thickness at the groove bottom subtread is approximately 2.0 to 4 mm so that the grooves gape greatly (i.e. high tensile stress is applied to the bottom of the grooves) upon inflation.
The recent trend, driven by the growing need for lightweight tires, has been towards improving process capability, such as the precision of the weight/profile of rubber applied, the precision of the application position, and roundness, to further thin the subtread and sidewall layers. With the advances in production technology, the preparation of thin treads or sidewalls, which is conventionally impossible due to manufacturing defects, is being enabled. The preparation of thinner outer layer components has made us aware of the importance of antioxidants and waxes in inner-layer rubber components because the problems then arise that would not arise in outer rubber layers that are sufficiently thick (approximately 3 to 6 mm) since, in this case, the outer-layer rubber compounds make overwhelming contributions against cracks on the surface of the outer rubber layers.
Outer rubber layers containing much wax, in the fresh condition, are likely to turn white due to wax blooming. Wax is mainly formed of normal alkanes —(CH2) (molecular weight: 14), and has a molecular weight of approximately 420 [=(number of repeating alkane units) (=30)×(unit molecular weight) (=14)], which is approximately equivalent to that of the process oil TDAE. Thus, its migration rate to adjacent rubbers can be considered to be equivalent to that of oil. So far, however, the migration of wax has been considered only for base treads. Thus, when thinner outer rubber layers are used, a sufficient amount of wax cannot be ensured on the surface of the outer rubber layers in the fresh condition and during service, which leads to the problems that sufficient static ozone resistance and brown discoloration resistance cannot be ensured.